How did one unexpected X-ray image lead to DNA fingerprinting?

On 10 September 1984, the geneticist Alec Jeffreys was looking at an X-ray film of a DNA experiment in his lab at the University of Leicester. The image unexpectedly showed both similarities and differences between the DNA of several members of his technician's family. Within about half an hour, Jeffreys realized what he was looking at: a way to identify individuals from variations in their DNA. That unexpected image was the start of DNA fingerprinting.

The method relies on regions of DNA that repeat in highly variable ways from person to person, variable enough for the pattern to be unique to each individual. Jeffreys's lab was, for a while, the only place on Earth that could run the test, and requests poured in from around the world before the technique was commercialized in 1987.

Its first real-world use came in 1985, in an immigration dispute over whether a British boy was truly related to the family he lived with. The DNA results proved the relationship, and Jeffreys later described seeing the relief on the mother's face when she heard the news. The technique's defining moment, though, came from a double murder. Two teenagers, Lynda Mann and Dawn Ashworth, had been raped and killed in Narborough, Leicestershire, in 1983 and 1986. Police tested blood from thousands of local men. One suspect, Richard Buckland, had already confessed, but his DNA didn't match the crime scene samples, and he was exonerated. The true killer, Colin Pitchfork, had tried to cheat the test by coercing a coworker into giving blood in his place, using a forged passport. The scheme unraveled when another coworker reported it, and Pitchfork, caught by the first police forensic use of DNA fingerprinting, was convicted in January 1988.

Without Jeffreys's method, authorities believe an innocent man would likely have spent his life in prison for a crime he didn't commit.